The present invention relates to the encoding and decoding of video information and, more particularly, to a method and system for secure transmission of television signals for subscription television or similar video services in which only authorized viewers are permitted to view a video program.
With the increased interest and activity in the field of subscription or pay video transmissin of all types (e.g., broadcast and cable subscription television, long distance satellite transmission, television transmission of textual information, etc.), there has arisen a need for more secure transmission of high quality video information. To this end, numerous video encoding or scrambling techniques have been developed and some are now in use, particularly in broadcast systems where there is no control over who receives the signals and thus the signals must be encoded to prevent unauthorized use of the received signals.
One broadcast subscription television system now in use in Los Angeles transmits a video signal that has been modulated by a sine wave signal such that the blanking and synchronizing levels cannot be recognized by a normal television receiver. The display, without decoding, is thus unintelligible or at least very annoying to a viewer. However, it has been reported that by rather simple means available in most electronics stores, the system may be readily defeated by non-subscribers. As this becomes more widely known to the public, the number of unauthorized viewers grows and the incentive to pay for the services diminishes. This, in turn, detracts from the desirability on the part of video program producers to permit the use of their programming, particularly if they are paid as a function of authorized viewers or revenues collected.
Other approaches to television signal scrambling have proven more secure and may, in fact, make it practically impossible to unscramble the video signals without highly sophisticated and extremely expensive equipment. One such approach is to invert lines or fields of video information on some basis that can be reproduced at the subscriber location to permit viewing of a normal picture. Thus, for example, one known system inverts alternate parts of the video information and a decoder at the subscriber location can reinvert those inverted portions to reconstitute the original video. Another known system inverts fields of video information on a random basis and sends a code with the scrambled video to instruct the decoder as to how the received video has been inverted.
Security tends to be adequate in approaches to video scrambling in which the video information is randomly inverted and a secure code is transmitted with the video so that the decoder can properly reinvert, but difficulties arise with respect to picture quality. For example, inversion and reinversion of video signals may result in a reconstituted video signal that varies in d.c. level from line-to-line or field-to-field. Because of this variation, a flicker or other annoying effect appears in the television display making it unpleasant for viewing.
Various measures have been employed to eliminate or at least reduce this problem with varying degrees of success. Clamping the video signals to the same d.c. level has been somewhat successful, but some annoying effects may still remain. Also, the additional circuitry required to eliminate or reduce the annoying effects of these types of scrambling add cost and complexity to the decoders. Less expensive and perhaps more effective approaches, such as less frequent inversion, have been suggested, but they seem to have a tendency to reduce the security of transmission and only serve to reduce the annoyance, not eliminate it.